Filter membranes for physiologically active substances

ABSTRACT

Filter membranes for efficiently eliminating pathogens such as viruses from solutions of drugs or physiologically active substances employed as starting materials thereof which are contaminated with pathogens such as viruses. These filter membranes, which can simultaneously achieve favorable performance of eliminating small viruses and a high physiologically active substance-permeability, can be obtained by controlling the membrane characteristics to give a ratio (BP/γ) of the bubble point BP (MPa) to the surface tension γ (N/m) of 110 or above, and/or logarithmic elimination ratios of swine parvovirus at 0 to 5 l/m 2  filtration volume and at 50 to 55 l/m 2 , and regulating the permeability for bovine immunoglubulin, wherein the monomer content amounts to at least 80%, to 70% or more. These filter membranes make it possible to provide drugs or starting materials therefor with little fear of the contamination with viruses, etc. and, therefore, are useful in the fields of pharmacy, medicine and the like.

TECHNICAL FIELD

The present invention relates to a filter membrane used to effectivelyremove pathogens such as viruses from solutions of medicinal products orphysiologically active products used as the raw materials thereof.

BACKGROUND ART

In the purification process of plasma derivatives or biopharmaceuticalproducts, technology for preventing virus infection, which may be causedby administration of products, has been used. A method of inactivatingor removing viruses is employed as this type of technology. As examplesof the method of inactivating viruses, a heat treatment method and achemical treatment method (Solvent/Detergent (S/D) treatment, forexample) can be given. As examples of the method of removing viruses, amembrane filtration method can be given. In the membrane filtrationmethod, particles are separated by size exclusion based on a sievingprinciple. Therefore, viruses can be removed only by size irrespectiveof chemical or thermal characteristics thereof. Because of this, themembrane filtration method using a virus removal membrane has beenwidely put into practical use on an industrial scale.

The heat treatment method exerts little effect on heat-resistant humanparvovirus B19, hepatitis A virus, and the like. The S/D treatmentmethod has essentially no effect on human parvovirus B19, poliovirus,reovirus, and SV-40 having no lipid envelope. In particular, since humanparvovirus B19 is heat resistant and has no lipid envelope, the virusremoval membrane is effective for inactivating or removing humanparvovirus B19.

In the purification process of plasma derivatives or biopharmaceuticalproducts, it is necessary to increase removability or inactivationcapability of viruses and permeability for physiologically activeproducts at the same time.

Virus removal membranes available at present are either a membrane whichallows high-molecular-weight physiologically active products such ashuman immunoglobulin and Factor VIII to pass therethrough, but exhibitsinferior small virus removal performance, or a membrane which can removesmall viruses, but cannot allow high-molecular-weight physiologicallyactive products such as human immunoglobulin or Factor VIII to passtherethrough at a practical level.

Specifically, conventional virus removal membranes are either a membranewhich allows high-molecular-weight physiologically active products suchas human immunoglobulin or Factor VIII to pass therethrough, but cannotremove small viruses such as human parvovirus B19, or a membrane whichcan remove small viruses such as human parvovirus B19, but cannot allowhigh-molecular-weight physiologically active products such as humanimmunoglobulin or Factor VIII to pass therethrough at a practical level.

Japanese Patent Application Laid-open No. 7-265674 discloses apolyvinylidene fluoride membrane capable of effectively removing smallparticles from liquid and exhibiting minimum adsorption, for which anintegrity test before actual use can be employed. Although the inventorsclam that this membrane is useful for removing viruses from a solution,its capability of allowing high-molecular-weight physiologically activeproducts to pass therethrough with high permeability is still unconcern.

Japanese Patent No. 1873816 and U.S. Pat. No. 4,808,315 disclose polymerporous hollow fiber membranes. These hollow fiber membranes have aspecific micropore structure effective for removing viruses from asolution of physiologically active products. These hollow fibermembranes are characterized by the specific micropore structureexhibiting superior virus removability and high permeability forphysiologically active products at the same time. However, these patentsneither describe nor suggest whether or not the membranes are effectivein the case of sieving high-molecular-weight physiologically activeproducts such as human immunoglobulin or Factor VIII from small virusessuch as human parvovirus B19 or poliovirus.

Japanese Patent Application Laid-open No. 4-505579 discloses a membranefor separating viruses from a solution. This membrane is a compositeasymmetric membrane which selectively separates viruses from a solutionwhich includes viruses. This membrane exhibits≧3 log reduction value forbacteriophage Φ×174 (28 nm) and ≧3 log reduction value for small virusessuch as human parvovirus B19 or poliovirus. However, the membraneexhibits an extremely low human immunoglobulin permeability of 10-20%and, therefore, cannot be used in practice.

Conventional filter membranes which selectively separate physiologicallyactive products from small viruses such as human parvovirus B19 have aproblem whereby virus removability decreases as the volume of filtrationis increased during continuous filtration. In particular, filtermembranes excelling in initial virus removability have a problem wherebyvirus removability suddenly decreases accompanied by an increase in thevolume of filtration.

DISCLOSURE OF THE INVENTION

The present invention has been achieved to solve the above problems inthe prior art. Specifically, an object of the present invention is toprovide a filter membrane for solutions of medicinal products orphysiologically active products used as the raw materials of medicinalproducts which may be contaminated with viruses, which can allow thephysiologically active products to pass therethrough at a practicallevel and remove small viruses such as human parvovirus B19 orpoliovirus, of which the characteristics can be maintained withoutsubstantial change depending on the volume of filtration. As a result,another object of the present invention is to provide technology forproviding safer products.

The present inventors have conducted extensive studies in order toachieve the above object to attain the present invention.

Specifically, according to one aspect, the present invention provides afilter membrane for solutions of physiologically active products, ofwhich the ratio (BP/γ) of a bubble point BP (MPa) to surface tension γ(N/m) is 110 or more and permeability for bovine immunoglobulin with amonomer content of 80% or more is 70% or more.

According to another aspect, the present invention provides a filtermembrane for solutions of physiologically active products, of which thelog reduction value for porcine parvovirus are 3 or more at both 0-5l/m² filtration and 50-55 l/m² filtration, and permeability for bovineimmunoglobulin with a monomer content of 80% or more is 70% or more.

According to still another aspect, the present invention provides afilter membrane for solutions of physiologically active products, ofwhich the BP/γ ratio is 110 or more, the log reduction value for porcineparvovirus are 3 or more at both 0-5 l/m² filtration and 50-55 l/m²filtration, and permeability for bovine immunoglobulin with a monomercontent of 80% or more is 70% or more. In the pore structure of thefilter membrane, the thickness of a region with a pore size logarithmicstandard deviation σ_(g) of 2.0 or less is preferably 3-90 μm. Thepurified water permeation rate of the filter membrane is preferably70-200 l/h/0.1 MPa per m² of the membrane area. The filter membranepreferably has no skin layer on the surface. The thickness of the filtermembrane is preferably 5-100 μm.

The pore size characteristics of the membrane of the present inventionmay be represented by the BP/γ ratio and/or the log reduction value forporcine parvovirus, and permeability for bovine immunoglobulin with amonomer content of 80% or more.

The BP/γ ratio and the log reduction value for porcine parvovirus areindexes regarding to the pore structure of the filter membrane. The BP/γratio relates to the pore structure at early stage of filtration. Thelog reduction value for porcine parvovirus is an index relating not onlyto performance at early stage of filtration, but also to the durabilityof filtration performance over time.

The virus concentration in the filtrate may vary depending on the volumeof filtration. In the case of a membrane having a small capacity forcapturing viruses, the log reduction value for porcine parvovirusdecreases as the volume of filtration is increased. A membraneexhibiting no or only a small decrease in the log reduction value forporcine parvovirus has a large capacity for capturing viruses. Thecapacity for capturing viruses increases as the volume of the regionwith a certain degree homogeneity in the pore structure of the membraneis increased. This causes the membrane to have excellent durability ofthe log reduction value for porcine parvovirus over time. In the presentinvention, the log reduction value for porcine parvovirus of 3 or moreat both 0-5 l/m² filtration and 50-55 l/m² filtration are indexesindicating the degree of durability, pore structure, capacity forcapturing viruses, and homogeneity of the membrane. An interfacialdestruction phenomenon is used to specify the pore structure of thefilter membrane. A bubble point test is used as a convenient method fordetermining the maximum pore size of the membrane. This method has beenused by Bechold et al. (H. Bechold et al., Kolloid Z., 55, 172 (1931),JIS K3832).

In this method, the membrane is wetted using liquid with a surfacetension of γ (N/m). When pressure is gradually applied to the membraneusing gas, continuous bubbling occurs at the surface of the membraneunder a specific gas pressure. This gas pressure is called the bubblepoint BP (MPa).

Any conventional measuring methods determine the pressure under whichoccurrence of continuous bubbling is confirmed, by naked eyeobservation, as the bubble point. However, in the case where the area ofthe membrane is small, occurrence of bubbles may be overlooked since theamount of bubbling is small. Moreover, separation of bubbles which haveattached to the surface of the membrane before applying pressure (whichare not produced by interfacial destruction phenomenon) may be mistakenfor the separation of bubbles produced by the interfacial destructionphenomenon. Therefore, errors tend to occur in those methods.

In the present invention, the pressure (MPa) under which continuousbubbling quantitatively occurs in an amount of 3.0 ml/min per cm² isdefined as the bubble point BP in order to reduce measurement errors.

The present inventors have found a correlation between the BP/γ ratioand the removal of human parvovirus B19. In more detail, the presentinventors have found that a membrane having pore size characteristicswith a BP/γ of 110 or more can remove human parvovirus B19 efficiently.

A method of analyzing particle removal performance is used to indirectlyspecify the pore structure of the filter membrane. The removalperformance is generally expressed by using the reduction value of modelparticles having the average particle size which is close to the averagepore size of the membrane and a particle size distribution as narrow aspossible.

In the technical field within the present invention, specific viruses orphages are used as particles having the above characteristics. Porcineparvovirus has an average particle size of 20-25 nm, has an extremelynarrow particle size distribution in a non-aggregated state, and has norisk of infection to human. Therefore, porcine parvovirus is a suitablemodel particle in the field to be applied the present invention.

In the present invention, the reduction value of human infectious smallviruses such as human parvovirus B19 may be estimated based on the BP/γratio. Moreover, in the present invention, the reduction value of humaninfectious small viruses such as human parvovirus B19 may also beestimated from the reduction value for porcine parvovirus which hassimilar particle size and other characteristics to those of humanparvovirus B19.

The reduction value for porcine parvovirus is expressed by the logreduction value (LRV) and can be calculated using the followingequation.

LRV=log₁₀(N ₀ /N _(f))

N₀: the number of porcine parvovirus in the feed solution

N_(f): the number of porcine parvovirus in filtrate

In the present invention, the log reduction value for porcine parvovirusmust be 3 or more at both 0-5 l/m² filtration and 50-55 l/m² filtration.

The virus filtration is performed under the condition of a pressure of0.0785 MPa and a temperature of 25° C. by constant pressure dead-endfiltration. The virus concentration in the filtrate may vary dependingon the volume of filtration. In the present invention, ≧3 LRV forporcine parvovirus means that both LRV of the filtrate at 0-5 l/m²filtration and LRV of the filtrate at 50-55 l/m² filtration are 3 ormore.

A membrane of which the LRV decreases as the volume of filtration isincreased has a small capacity for capturing viruses. A membrane ofwhich the LRV does not decrease, or decreases to only a small extent hasa large capacity for capturing viruses. Therefore, the LRVs at the twopoints can indicate the membrane structure characteristics.

It is difficult to produce a membrane which can allow solution ofphysiologically active products to pass therethrough at a practicallevel, and excels in LRV durability in the removal of small viruses suchas human parvovirus B19 or poliovirus using a conventional technique.

The membrane of the present invention excels in LRV durability even inthe removal of small viruses due to increased capacity of the regionwith a certain degree of homogeneity in the pore structure of themembrane. As the feed solution for determining the log reduction valuefor porcine parvovirus, a culture supernatant obtained after culturingESK cells (pig kidney cells) infected with porcine parvovirus inDulbecco's MEM medium containing 3% fetal bovine serum was used.

The concentration of porcine parvovirus in the solution beforefiltration and in the filtrate was determined by a TCID₅₀ methodutilizing agglutination of chicken erythrocyte after culturing eachsolution added to the ESK cells for 10 days, respectively.

There is no established general technique (assay) for measuring theconcentration of human parvovirus B19 by observation of celldegeneration or the like. Concentration measurement (assay) using a PCRmethod may be used in some cases. However, it is difficult to obtainprecise data on the virus reduction value due to insufficientsensitivity. Therefore, effectiveness of the method of removing orinactivating human parvovirus B19 is estimated by evaluation usingporcine parvovirus or canine parvovirus, for which high sensitivityassay by the observation of cell degeneration are established. Actually,a technique, which is determined effective for inactivating or removinghuman parvovirus B19 by evaluation using porcine parvovirus or canineparvovirus, exhibits practical performance in the process for producingproducts.

In the membrane filtration of physiologically active products, adecrease in permeability means an increase in the degree of clogging thepore structure of the membrane by the physiologically active products.Clogging the pore structure of the membrane causes an increase in theloss of physiologically active products captured in the membrane, adecrease in the concentration of physiologically active products in thefiltrate, a decrease in the filtration volume per unit area of themembrane, and the like, thereby increasing costs in the process forproducing products. Therefore, the practical level of permeability forphysiologically active products during membrane filtration step in theindustrial process for producing products is 70% or more, and preferably80% or more.

Permeability for physiologically active products varies depending ontypes of substances and properties of the solution. Human immunoglobulinhas a molecular weight of 160,000-900,000, which is generally thegreatest range for physiologically active products put into practicaluse in the fields of medicine and medication. Moreover, humanimmunoglobulin has a high aggregation property to a great extent.Therefore, it seems to be difficult to improve permeability for humanimmunoglobulin.

In the process for producing plasma derivatives using human blood as theraw material, human blood is usually subjected to a plurality ofpurification processes such as Cohn fractionation, in which proteincomponents of blood plasma are fractionated by utilizing the differencein affinity with ethanol, and chromatography. The resulting humanimmunoglobulin is subjected to virus removal using a filter membrane.

The content of contaminants or polymers in human immunoglobulin beforefiltration is smaller than that in bovine immunoglobulin. Therefore, amembrane exhibiting high permeability for bovine immunoglobulin iseasily estimated to exhibit the same or higher permeability for humanimmunoglobulin. The membrane of the present invention, of which thepermeability for bovine immunoglobulin with a monomer content of 80% ormore is 70% or more, brings significant effects during filtration ofmedicinal products or physiologically active products used as the rawmaterials of medicinal products from the viewpoint of costs in theprocess for producing products.

Permeability for bovine immunoglobulin was calculated as follows.

Permeability for bovine immunoglobulin=C _(f) /C ₀×100

C_(f): Bovine immunoglobulin concentration before filtration (feedsolution)

C₀: Bovine immunoglobulin concentration after filtration (filtrate)

Filtration for calculating the permeability for bovine immunoglobulin isperformed under a pressure of 0.0785 MPa at a temperature of 25° C. byconstant pressure dead-end filtration.

As the feed solution of bovine immunoglobulin, a solution prepared bydiluting a bovine immunoglobulin solution (manufactured by LifeTechnology) with 0.15N NaCl to a concentration of 3 wt % and removingcontaminants by prefiltration using PLANOVA 35N (manufactured by AsahiKasei Corporation, formerly Asahi Chemical Industry Co., Ltd.) was used.The molecular weight distribution of bovine immunoglobulin in theunfiltered solution was measured by liquid chromatography. As a result,the monomer content was 80% or more.

This feed solution was filtered for three hours using a separationmembrane to obtain the filtrate.

The bovine immunoglobulin concentration in the feed solution and in thefiltrate was calculated by measuring the absorbance at 280 nm using a UVspectrophotometer.

In the case of separating small particles such as physiologically activeproducts and large particles such as viruses by using a virus removalmembrane based on the sieving principle, pores with a pore sizeintermediate between the diameters of these two types of particles aresubstantially effective. In order to allow the pores having a diameterin such a range to exhibit sufficient effectiveness, it is inevitablefor the pore structure of the membrane to have a region with a certaindegree of homogeneity. Specifically, in the pore structure of themembrane of the present invention, the thickness of the region in whichthe pore size logarithmic standard deviation σ_(g) is 2.0 or less ispreferably 3-90 μm, and still more preferably 15-50 μm. Said homogeneitymay be directly measured by pore size measurement using an electronmicroscope.

The membrane of the present invention having the above pore structurewherein exhibits a higher log reduction value for porcine parvovirus notonly in the initial stage of filtration but also subsequent filtration,and simultaneously allows bovine immunoglobulin to pass therethrough ata higher rate, because of the increased capacity of the region with acertain degree of homogeneity.

The greater the thickness of the region in which the pore sizelogarithmic standard deviation σ_(g) is 2.0 or less, the higher the logreduction value for porcine parvovirus. However, an excessive increasein the thickness gives rise to a disadvantage with respect topermeability for bovine immunoglobulin.

The pore size logarithmic standard deviation σ_(g) is calculatedaccording to the following equation.

lnσ _(g)=((ΣΔn _(i)(lnD _(pi) −lnD _(pg))² /N)^(1/2)

lnD _(pg) =ΣΔn _(i) lnD _(pi) /N

Δn_(i): Number of pores with a pore size of D_(pi)

D_(pi): Pore size (nm)

D_(pg): Logarithmic average pore size (nm)

N: Total number of pores

The pore size of the membrane is measured through an electronmicroscope. The membrane is embedded in a polymer resin such as anacrylic resin. The embedded membrane is cut into a thin piece using aconventional method so that the lateral cross section of the membrane isexposed. The cut cross section of the membrane is photographed using ascanning electron microscope (SEM). The obtained photograph is analyzedby image processing. The cross section of the membrane is divided in thedirection of the thickness. The pore size (D_(pi)) and the number ofpores (Δn_(i)) are determined for each divided region. The pore sizereferred herein means the diameter converted the pore shown in the SEMphotograph into a real circle.

In the present invention, the suitable purified water permeation rate ispreferably 70-200 l/h/0.1 MPa, and still more preferably 90-120 l/h/0.1MPa per m² of the membrane area. Allowing the purified water permeationrate to which all the pores in the membrane contribute to be 70 l/h/0.1MPa or more per m² of the membrane area enables the amount of filtrationand permeability for solution of physiologically active products to beincreased to a practical level. This is particularly advantageous in theincrease in the volume of filtration of the solution of physiologicallyactive products. If the purified water permeation rate to which all thepores in the membrane contribute exceeds 200 l/h/0.1 MPa per m² of themembrane area, it is difficult to increase the log reduction value forporcine parvovirus to 3 or more at both 0-5 l/m² filtration and 50-55l/m² filtration.

The purified water permeation rate referred herein means the value shownas the flow rate of purified water filtered under a trans-membranedifferential pressure of 0.1 MPa at a temperature of 37° C. in the unitof l/h/0.1 MPa per m² of the membrane area (in a dry state). Purifiedwater referred herein means water purified by ultrafiltration.

The skin layer referred in the present invention means an extremely thinlayer present on one side or both sides of the membrane, which has afine structure in comparison with the inside of the membrane. Generally,a membrane of which filtration characteristics are owed to only the skinlayer may also achieve a purified water permeation rate of 70 l/h/0.1MPa per m² of the membrane area or more. However, such a membrane hardlyachieve≧3 LRV for porcine parvovirus at both 0-5 l/m² filtration and50-55 l/m² filtration. This is because the skin layer inevitably hasdefects such as pinholes or cracks, thereby resulting in unreliabilityrelating to the virus reduction value.

The suitable thickness of the membrane of the present invention is 5-100μm, and preferably 20-100 μm. In the production of the membrane of thepresent invention, the coagulation rate inside the membrane greatlyvaries depending on the distance from the surface of the membrane.Therefore, if the thickness of the membrane exceeds 100 μm, it isdifficult to control the pore structure of the membrane. Therefore, thethickness of the membrane is preferably 100 μm or less. The membranehave need of a certain degree of thickness in order to ensure ≧3 LRV forporcine parvovirus at both 0-5 l/m² filtration and 50-55 l/m²filtration. Therefore, the thickness of the membrane is preferably 5 μmor more.

The membrane referred in the present invention is a virus removalmembrane for removing viruses, an ultrafilter membrane, microfiltrationmembrane, or the like. As the material for the membrane, regeneratedcellulose, polyvinylidene fluoride, polysulfone, polyacrylonitrile, andthe like can be given, but the materials other than those materials maybe included. The form of the membrane may be any of a hollow fibermembrane, flat membrane, pleat membrane, and spiral membrane. Inaddition, the membrane may be a composite membrane in which membranesare layered.

The physiologically active products referred in the present inventionmeans physiologically active products generally used in the fields ofmedicine, medication, and diagnosis reagents. Specific examples includeproteins, polypeptides, polysaccharides, combinations thereof, and thelike. The origin of these physiologically active products is human,animal, or cultured cells. These physiologically active products includephysiologically active products produced by cultured animal cells usinggenetic recombination or cell fusion technique, physiologically activeproducts produced by secretory tissues of animals using a transgenictechnique, and the like.

As examples of proteins, blood coagulation factors such as F-IX, F-XI,F-VIII, F-VII, fibrinogen, thrombin, antithrombin-III, and mixturesthereof, human immunoglobulin such as IgG, IgA, IgD, IgE, and IgM,alubumin, α-1 protease inhibitor, trypsin inhibitor, protease inhibitor,streptokinase, apolipoprotein, and growth factors, and the like can begiven. As examples of polypeptides, physiologically active polypeptidessuch as recombinant human growth hormone produced using mammal cells,protease inhibitor originating from bovine tissue, and the like can begiven. As examples of polysaccharides, glycosaminoglycans such asheparin, heparin fragments, heparin derivatives, heparan sulfate, andhyaluronic acid can be given.

Human immunoglobulin products generally exhibit low permeability duringmembrane filtration due to the high concentration in the solution.Therefore, it is difficult to remove human parvovirus B19, poliovirus,or the like while allowing human immunoglobulin products to pass throughthe membrane. However, the membrane of the present invention can besuitably used for human immunoglobulin products due to high permeabilityand the high reduction value of human parvovirus B19, poliovirus, or thelike. Therefore, the present invention exhibits especially significanteffect in the case where the physiologically active products are humanimmunoglobulin.

The solution of physiologically active products is preferably filteredunder conditions in which clogging is hard to occur taking permeabilityinto consideration. It is also practically preferable from the viewpointof economy.

There are no specific limitations to the protein concentration in thecase of using human immunoglobulin. The protein concentration ispreferably 5 wt % or less, and still more preferably 3 wt % or less forpractical use.

As a method for producing the membrane of the present invention usingvarious types of polymers, non-solvent induced phase separation orthermally induced phase separation is generally used. The objectivemembrane structure can be produced by adjusting the raw material polymerconcentration in the polymer solution and equilibrium factors andkinetics factors in chemical changes during phase separation andcoagulation.

More specifically, in the case of producing a hollow fiber membraneusing a double spinning nozzle, the membrane structure varies dependingon chemical equilibrium factors and kinetic factors during membraneproduction such as the polymer concentration in the spinning solution,inner solution composition, outer solution composition, extruding rateof the spinning solution, and winding rate of the membrane. If theextruding rate and the winding rate are low, the pore size distributionbecome narrow and the thickness of the effective region of the membranehaving a homogeneous pore size is increased. However, an excessivelowering in the extruding rate and the winding rate results in adecrease in production efficiency, and is not practical. Therefore, inthe case of setting the extruding rate and the winding rate to practicalconstant values, the membrane structure can be changed by adjusting thepolymer concentration in the spinning solution, inner solutioncomposition, and outer solution composition. If the non-solventconcentration in the inner solution and the outer solution is decreased,the pore size distribution of the region inside the membrane whichsubstantially contributes separation can be made narrower and thethickness of this region can be increased. Therefore, the BP/γ ratio andthe log reduction value for porcine parvovirus tend to be increased.

The method for producing the filter membrane of the present invention isdescribed below in detail taking a hollow fiber membrane formed ofcuprammonium regenerated cellulose as an example. A cuprammoniumcellulose solution and an aqueous solution including a non-solvent whichcauses micro-phase separation (hereinafter called “coagulatingsolution”) are prepared using a conventional method (Japanese PatentApplications Laid-open No. 59-204912 and No. 59-204911, for example).Specifically, cellulose is dissolved as the raw material of the membranein a cuprammonium solution to prepare a cuprammonium cellulose solutionwith a cellulose concentration of about 7.0-8.0 wt %. The coagulatingsolution comprises two kinds of solutions of an outer solution which isallowed to act from outside the hollow fiber and an inner solution whichis introduced into the hollow fiber and allowed to act therefrom. Thecomposition of the outer solution preferably has an acetoneconcentration of about 20-35 wt % and an ammonia concentration of about0-0.1 wt %. The composition of the inner solution preferably has anacetone concentration of about 30-50 wt % and an ammonia concentrationof about 0-0.5 wt %. This example illustrates a non-solvent inducedmembrane production method using acetone as the non-solvent. It ispreferable to decrease the concentration of the non-solvent which causesmicro-phase separation in the coagulating solution. This is because thepore size distribution in the region inside the membrane can be madenarrower, and the thickness of this region can be further increased bydecreasing the non-solvent concentration.

The cuprammonium cellulose solution and the coagulating solutionprepared as described above are subjected to spinning, coagulation,regeneration, washing with water, and drying under vacuum using a methoddisclosed in Japanese Patent Application Laid-open No. 4-371221 toobtain a hollow fiber membrane.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is described below by examples, which should notbe construed as limiting the present invention.

EXAMPLES 1-3

A cuprammonium cellulose solution and a coagulating solution wereprepared using a method disclosed in Japanese Patent ApplicationLaid-open No. 59-204912. A hollow fiber membrane was produced using amethod disclosed in Japanese Patent Application Laid-open No. 4-371221.Specifically, cotton linters (average molecular weight: 1.44×10⁵) weredissolved in a cuprammonium solution prepared using a conventionalmethod to prepare a spinning solution with a cellulose concentration of7.5 wt %. The spinning solution was extruded from an outer nozzle of acircular double spinneret. At the same time, an inner solution having acomposition shown in Table 1 was extruded from a center nozzle of thedouble spinning nozzle. The extruding rate of the spinning solution isshown in Table 1. The solutions were extruded into an outer solutionhaving a composition shown in Table 1 and winded. The winding rate was10 m/minute.

The pore structure of the membrane can be controlled by adjusting thecellulose concentration, outer solution composition, and inner solutioncomposition, whereby the membrane of the present invention can beobtained.

A U-shaped narrow tube disclosed in Japanese Patent ApplicationLaid-open No. 4-371221 was used as a coagulation bath in thiscoagulation step. The winded hollow fiber membrane was subjected toregeneration with dilute sulfuric acid solution, washing with water, anddrying under vacuum using a method disclosed in Japanese PatentApplication Laid-open No. 4-371221. The hollow fiber membrane thusobtained was assembled into a filter by a conventional method using apolyurethane sealant.

The inner diameter, thickness, BP/γ ratio, purified water permeationrate, porcine parvovirus LRV, and bovine immunoglobulin permeability ofthe resulting hollow fiber membrane are shown in Table 1. The bubblepoint, purified water permeation rate, porcine parvovirus LRV, andbovine immunoglobulin permeability were measured according to theabove-described methods. In the examples, the bubble point was measuredby producing low pressure test conditions measurable within the range ofwithstand pressure of the membrane using perfluorocarbon with surfacetension γ of 0.012 (N/m) as wetting liquid, and using nitrogen as a gas.

As a result of observation using an electron microscope, the membranesin Examples 1-3 had no skin layer on the surface.

TABLE 1 Example 1 2 3 Cellulose concentration (wt %) 7.5 7.5 7.5 Outersolution composition Acetone concentration (wt %) 30 25 30 Ammoniaconcentration (wt %) 0 0 0 Inner solution composition Acetoneconcentration (wt %) 40 45 40 Ammonia concentration (wt %) 0 0.65 0Extruding rate of spinning solution 3.65 3.65 3.00 (ml/min) Innerdiameter (μm) 404 414 396 Membrane thickness (μm) 32 30 25 BP/γ ratio(MPa/(N/m)) 120 135 115 Purified water permeation rate*¹⁾ 107 97 118Porcine parvovirus LRV*²⁾ 5.1 >6.0 4.1 Porcine parvovirus LRV*³⁾4.2 >6.0 3.1 Bovine immunoglobulin permeability (%) >90 >6.0 >90*¹⁾l/h/0.1 MPa per m² of membrane area *²⁾Volume of virus solutionfiltered: 0-5 (l/m²) *³⁾Volume of virus solution filtered: 50-55 (l/m²)

The porosity measured in the direction of the thickness and thelogarithmic standard deviation σ_(g) of the pore size distribution ofthe membrane in Example 1 are shown in Table 2. In the membrane inExample 1, the thickness of a region with a logarithmic standarddeviation σ_(g) of 2.0 or less was 20 μm (corresponding to 63% ofthickness of membrane).

TABLE 2 Region in thickness Logarithmic direction (%) Porosity (%)standard deviation  0-12 51 2.48 12-25 29 2.08 25-37 20 1.82 37-49 271.88 49-62 27 1.86 62-75 26 1.79 75-88 20 1.82  88-100 35 2.12

From the relation between human parvovirus B19 and porcine parvovirus,the membranes in Examples 1-3 can be estimated to exhibit virus removalperformance for human parvovirus B19 equal to that for porcineparvovirus. From the relation between bovine immunoglobulin and humanimmunoglobulin, high permeability relating to bovine immunoglobulinobtained in Examples 1-3 is easily estimated to be achieved even in thepractical process for producing human immunoglobulin preparations at thesame or higher level.

Specifically, the membranes in Examples 1-3 allow physiologically activeproducts to pass therethrough at a practical level and are capable ofremoving small viruses such as human parvovirus B19 or poliovirus fromsolutions of medicinal products or physiologically active products usedas the raw materials of medicinal products which have risk of thecontamination with viruses, therefore these are excellent membranescapable of providing safer products.

COMPARATIVE EXAMPLES 1-3

Hollow fiber membranes of Comparative Examples 1-3 were prepared in thesame manner as in Examples 1-3. The compositions of the cuprammoniumcellulose solution and the coagulating solution (inner and outersolutions) were as shown in Table 3.

The inner diameter, thickness, BP/γ ratio, purified water permeationrate, porcine parvovirus LRV, and bovine immunoglobulin permeability ofthe resulting hollow fiber membrane are shown in Table 3.

The bubble point, purified water permeation rate, porcine parvovirusLRV, and bovine immunoglobulin permeability were measured according tothe above-described methods. In the comparative examples, the bubblepoint was measured by producing low pressure test conditions measurablewithin the range of withstand pressure of the membrane usingperfluorocarbon having a surface tension γ of 0.012 (N/m) as wettingliquid, and using nitrogen as a gas in the same manner as described inthe above examples.

As is clear from the results shown in Table 3, the membranes inComparative Examples 1 and 3 are estimated to have high humanimmunoglobulin permeability, but have little capability for removingsmall viruses such as human parvovirus B19. The membrane in ComparativeExample 2 is estimated to be able to highly remove small viruses such ashuman parvovirus B19, but have no human immunoglobulin permeability at apractical level.

TABLE 3 Comparative Example 1 2 3 Cellulose concentration (wt %) 6.967.5 7.5 Outer solution composition Acetone concentration (wt %) 37.537.5 30 Ammonia concentration (wt %) 0.1 0.1 0 Inner solutioncomposition Acetone concentration (wt %) 53 45 53 Ammonia concentration(wt %) 0.65 0.65 0.65 Extruding rate of spinning solution 3.65 3.00 3.65(ml/min) Inner diameter (μm) 330 330 345 Membrane thickness (μm) 35 2634 BP/γ ratio (MPa/(N/m)) 69.6 137 105 Purified water permeation rate*¹⁾308 59 122 Porcine parvovirus LRV*²⁾ 0.2 >4.9 2.1 Porcine parvovirusLRV*³⁾ 0.1 >5.1 2.1 Bovine immunoglobulin permeability (%) 100 55 >95*¹⁾l/h/0.1 MPa per m² of membrane area *²⁾Volume of virus solutionfiltered: 0-5 (l/m²) *³⁾Volume of virus solution filtered: 50-55 (l/m²)

INDUSTRIAL APPLICABILITY

According to the membrane of the present invention, in the filtration ofsolutions of medicinal products or physiologically active products usedas raw materials of medicinal products which may be contaminated byviruses, the membrane can achieve superior performance for removingsmall viruses such as human parvovirus B19 or poliovirus (durability of≧3 LRV for human parvovirus B19, for example) and high permeationperformance for physiologically active products (human immunoglobulinpermeability of 70% or more, for example), thereby the present inventioncan also provide technologies for preparing safer preparations.

What is claimed is:
 1. A filter membrane for solutions ofphysiologically active products, comprising a filter membrane having aratio (BP/γ) of a bubble point BP (MPa) to surface tension γ (N/m) of110 or more and a permeability for bovine immunoglobulin with a monomercontent of 80% or more of 70% or more.
 2. A filter membrane forsolutions of physiologically active products, comprising a filtermembrane having a log reduction value for porcine parvovirus of 3 ormore at both a 0-5 l/m² filtration volume and a 50-55 l/m² filtrationvolume, and a permeability for bovine immunoglobulin with a monomercontent of 80% or more of 70% or more.
 3. A filter membrane forsolutions of physiologically active products, comprising a filtermembrane having a ratio (BP/γ) of a bubble point BP (MPa) to surfacetension γ (N/m) of 110 or more, a log reduction value for procineparvovirus of 3 or more at both a 0-5 l/m² filtration volume and a 50-55l/m² filtration volume, and a permeability for bovine immunoglobulinwith a monomer content of 80% or more of 70% or more.
 4. The filtermembrane according to any one of claims 1 to 3, wherein, in a porestructure of the filter membrane, the thickness of a region in which apore size logarithmic standard deviation σ_(g) is 2.0 or less is 3-90μm.
 5. The filter membrane according to any one of claims 1 to 3,wherein the filter membrane further has a purified water permeation rateof 70-200 l/h/0.1 MPa per m² of membrane area.
 6. The filter membraneaccording to any one of claims 1 to 3, wherein the filter membrane hasno skin layer on a surface thereof.
 7. The filter membrane according toany one of claims 1 to 3, wherein the filter membrane has a thickness of5-100 μm.